Preserving the value of glyphosate Bob Hartzler, Iowa State
University Mike Owen, Iowa State University
Chris Boerboom, University of Wisconsin Kevin Bradley, University of Missouri
Jeff Gunsolus, University of Minnesota Bill Johnson, Purdue University
Jim Kells, Michigan State University Mark Loux, Ohio State University
Aaron Hager, University of Illinois Dawn Nordby, University of Illinois
Christy Sprague, Michigan State University Bryan Young, Southern Illinois UniversityFebruary 20, 2004
- It is well known that glyphosate-resistant horseweed (also known as marestail)
populations have been selected in Roundup Ready soybean and cotton cropping
systems. Resistance was first reported in Delaware in 2000, a mere 5 years
after the introduction of Roundup Ready soybean. Since that initial report,
glyphosate-resistant horseweed is now reported in 12 states and is estimated to
affect 1.5 million acres in Tennessee alone.

A person could ask if this is
any indication of what might lie ahead. On one hand, it has been proposed in a
popular advertisement that glyphosate-resistant
weeds are unlikely to occur if glyphosate is frequently used, as long as
glyphosate is applied at full rates. The recommendations in this advertisement,
in part, are based on several long-term university studies of Roundup Ready
cropping systems. We feel a person should ask whether or not these studies can
prove that resistance will or won’t happen. It is our belief that these studies
are not large enough to test if resistance will develop. For example, the
University of Wisconsin has a 7 year Roundup Ready cropping system trial. This
trial has horseweed in the no-till plots. Despite burndown and in-crop
treatments with glyphosate, glyphosate-resistant horseweed has not developed in
this study. Since this trial did not find glyphosate-resistant horseweed, does
this mean that glyphosate-resistant horseweed cannot develop? Does it mean
that the resistant horseweed in Ohio or Tennessee or Delaware is not truly
resistant? Obviously not. Small scale trials cannot prove that some event will
not occur when a larger scale is considered. In total, these Roundup Ready
cropping system trials may only be testing continuous glyphosate use on perhaps
50 acres, which is an extremely small area compared to the total acres relying
on glyphosate as a primary management tool. The true trial to determine if a
rare event like glyphosate-resistant weeds will develop is actually being tested
on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that
are sprayed each year.

Many weed scientists across the
Midwest have warned of the potential for additional glyphosate-resistant weeds
if a “high selection pressure” is maintained. In this case, “high selection
pressure” refers to the repeated use of glyphosate without interruption by
herbicides with other modes of action or other weed management practices. This
potential was confirmed at the North Central Weed Science Society Meeting where
glyphosate-resistant common ragweed weed was reported. This is the first report
of glyphosate-resistance in common ragweed. It was identified in a Missouri
no-till soybean field that has been in continuous soybean production (with some
double crop wheat) for many years and in Roundup Ready soybean since 1996.
This field had a high selection pressure for glyphosate resistance with one or
more glyphosate applications per year. This is the second example of a
glyphosate-resistant weed developing in a Roundup Ready cropping system. We do
not know which weed species will be the next to develop glyphosate resistance or
when it will occur, but high selection pressure will likely result in additional
cases of resistance.

Midwest weed scientists
appreciate the value that glyphosate and Roundup Ready crops offer to growers.
We hope that growers and crop advisors will evaluate how they use glyphosate and
the Roundup Ready technologies to gain the value of these technologies without
increasing the risk of resistance.

Ideally, we recommend:
1)tank
mixing glyphosate with another mode of action like 2,4-D in burndown treatments
when glyphosate will be applied in the subsequent crop,
2)
alternating glyphosate use with other herbicide modes of action between years,
and3)
incorporating appropriate integrated weed management practices such as
cultivation.

Common chemical and
trade names are used in this publication. The use of trade names is for clarity by the
reader. Inclusion of a trade name does not imply endorsement of that particular brand of
herbicide and exclusion does not imply nonapproval.